Because the creds parameters are to be passed inside pie/restorer
code but read before thread_restore_args and task_restore_args
structures are allocated we need a small trick and prepare
creds int several stages

 - collect all creds data into separate private memory blobs
 - once all memory needed for restorer is allocated we relocate
   pointers in this blocks and setup
   thread_restore_args::thread_creds_args to appropriate
   address
 - restorer works as usual and setup creds parameters as before

v2:
 - fix addressing in positioning of rst_ memory (I've occasionally
   zap pointers and when been sending patches forgot to merge changes
   back, so while I've the series successfully restoring containers
   with different creds, if been merged the series won't work. So
   all changes are merged as appropriate)

 - drop module's global @cap_last_cap from pie/restorer.c
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>